The present invention relates to a multi-layer circuit board with at least two or more layers of circuit patterns connected, and a method of manufacturing same.
With a recent trend of electronic equipment becoming more miniaturized and higher in component density, there is an increasing demand for multi-layered circuit boards in the fields of both industrial and home-use equipment.
In the field of such multi-layer circuit board, there is a strong demand for the development of a method of making inner via-hole connection between multi-layered circuit patterns, and a circuit board having a highly reliable structure. As a method of manufacturing such circuit board, Japanese Laid-open Patent No. 6-268345 is proposing a method of manufacturing a high density circuit board having a novel structure wherein inner via-hole connections are made by conductive paste. This conventional method of manufacturing a circuit board will be described in the following.
A method of manufacturing a circuit board having four layers as a prior art multi-layer circuit board is described in the following.
First, a method of manufacturing a double-side circuit board being the base for a multi-layer circuit board is described.
FIG. 7 is a sectional view of the process for a conventional method of manufacturing double-side circuit boards for inner layers. In FIG. 7, the substrate is formed of prepreg sheet 21. The prepreg sheet 21 has a shape of 250 mm sq. and about 150 xcexcm thick. For example, the prepreg sheet 21 is formed of a composite material having non-woven fabric made from aromatic polyamide fiber and thermosetting epoxy resin impregnated to the non-woven fabric. Releasing films 22a, 22b have a plastic film and a Si type releasing agent applied to one side of the film, and the releasing films 22a, 22b are about 16 xcexcm in thickness. For example, as plastic film, polyethylene terephthalate is employed.
A method of bonding the prepreg 21 and releasing film 22a, 22b together is disclosed in Japanese Laid-open Patent No. 7-106760. Japanese Laid-open Patent No. 7-106760 refers to a method of continuously bonding releasing films 22a, 22b by dissolving the resin component of prepreg 21 with the use of a laminating apparatus.
A through-hole 23 is formed in the pregreg sheet 21 and releasing films 22a, 22b bonded together. The through-hole 23 is filled with conductive paste 24. Metallic foils 25a, 25b such as copper of 18 xcexcm thick are bonded to either side of the prepreg sheet 21. The conductive paste 24 is electrically connected to the metallic foil 25a, 25b. 
In FIG. 7,(a) the releasing films 22a, 22b are bonded to either side of the prepreg sheet 21. Next, (b) the through-holes 23 are formed, at predetermined portions, in the prepreg sheet 21 with the releasing films 22a, 22b bonded thereto, by using a laser beam machining process.
Next, (c) the conductive paste 24 is filled into the through-holes 23. In the method of filling the conductive paste 24, the prepreg sheet 21 having the through-holes 23 is placed on the table of a printer (not shown) and the conductive paste 24 is directly printed on the releasing film 22a. In this case, the releasing films 22a, 22b serve a function as a printing mask and also a function to prevent contamination of the prepreg sheet 21.
Subsequently, (d) the releasing films 22a, 22b are removed from either side of the prepreg sheet 21.
Next, (e) the metallic foils 25a, 25b are laminated to either side of the prepreg sheet 21. And the laminated metallic foils 25a, 25b and prepreg sheet 21 are heated under pressures at a temperature of about 200xc2x0 C. and pressure of about 4 MPa for one hour in a vacuum.
In this way, (f) the prepreg sheet 21 is compressed and becomes reduced in thickness (t2) to approximately 100 xcexcm. At the same time, the prepreg sheet 21 and metallic foils 25a, 25b are bonded to each other. Further, the metallic foil 25a disposed on the surface side and the metallic foil 25b disposed on the back side are electrically connected to each other by the conductive paste 24 filled in the through-holes 23 formed at the predetermined positions.
After that, the metallic foils 25a, 25b are selectively etched, and then circuit patterns 31a, 31b are formed respectively on either side thereof. Thus, a double-side circuit board can be obtained.
FIG. 8 is a sectional view of the process for a conventional method of manufacturing a multi-layer circuit board, and the multi-layer circuit board is a four-layer circuit board.
In FIG. 8(a), a double-side circuit board 40 having circuit patterns 31a, 31b manufactured by the steps from (a) to (g) of FIG. 4 and prepreg sheets 21a, 21b with conductive paste 24 filled in through-holes 23 manufactured by the steps from (a) to (d) of FIG. 7 are prepared.
Next, as shown in FIG. 8(b), metallic foil 25b, prepreg sheet 21b, inner layer double-side circuit board 40, prepreg sheet 21a, and metallic foil 25a are positioned and laminated in this order.
Subsequently, the laminated board formed of these is heated under pressures at a temperature of about 200xc2x0 C. and pressure of about 4 MPa for one hour in a vacuum, thereby curing the prepreg sheets 21a, 21b. Thus, as shown in FIG. 8(c), the prepreg sheets 21a, 21b are compressed and become reduced in thickness (t2) to 100 xcexcm, then the double-side circuit board 40 and metallic foils 25a, 25b are bonded to each other. The circuit pattern 31a and circuit pattern 31b of the double-side circuit board 40 are connected to the metallic foils 25a, 25b by inner via-holes filled with conductive paste 24. Next, as shown in FIG. 8(d), the metallic foils 25a, 25b are selectively etched, thereby forming the circuit patterns 32a, 32b. In this way, a circuit board having four layers can be obtained.
However, in the above conventional method of manufacturing a circuit board, a prepreg sheet formed of half-cured resin are heated under pressures to bond the prepreg sheet to a metallic foil and also to harden the prepreg sheet. In this case, during heating under pressures, a cushion is generally used as an intermediate material to compensate for variation in thickness of the prepreg sheet. However, due to variation in quantity and fluidity of the resin contained in the prepreg sheet, the peripheral resin excessively flows during heating under pressures, resulting in insufficient application of the pressure to the central portion of the substrate, and accordingly, the conductive paste will not be sufficiently compressed. As a result, there has been a problem of fluctuation in connection resistance.
A multi-layer circuit board of the present invention comprises:
(a) a double-side circuit board including a first substrate, a plurality of first through conductors disposed in the first substrate, bonding layers disposed on either side of the first substrate, and a first circuit pattern and a second circuit pattern disposed on the surface of the bonding layer,
wherein the first circuit pattern and the second circuit pattern are electrically connected to each other by the first through conductors;
(b) an intermediate substrate including a second substrate, a plurality of second through conductors disposed in the second substrate, and a first bonding layer and a second bonding layer disposed on either side of the second substrate,
wherein the first bonding layer and the second bonding layer are disposed on either side of the substrate except the second through conductors, and
the intermediate substrate is disposed on the surface of at least one of the first circuit pattern and the second circuit pattern.
By the above configuration, the through conductors disposed between the respective circuit patterns become uniform in connection resistance, and consequently, a circuit board having a stable connection resistance can be obtained.
A method of manufacturing a circuit board of the present invention comprises the steps of:
(a) manufacturing an intermediate substrate, including the steps of:
(i) forming a substrate having incompressibility;
(ii) forming bonding layers on either side of the substrate;
(iii) forming a plurality of through-holes in the substrate; and
(iv) filling conductive paste into the through-holes,
(b) manufacturing a double-side circuit board, including the steps of:
(v) laminating metallic foils to either side of the intermediate substrate and heating same under pressures, and
(vi) forming a first circuit pattern and a second circuit pattern by processing the metallic foils.
By the above configuration, the conductors electrically connecting the first circuit pattern and the second circuit pattern to each other become uniform in connection resistance, and consequently, a circuit board having a stable connection resistance can be obtained.